While refrigerators are running, their coolers, also called evaporators of the refrigerators, are frosted and often get ice condensed on the coolers. Such frost and ice must be removed periodically for a smooth operation of the refrigerator. For this purpose, many refrigerators have defrost timers. The defrost timer turns off periodically a compressor of the refrigerator so that it allows the temperature of the evaporator to be high enough to melt and dry the ice and frost formed on the evaporator.
Since refrigerators are ubiquitous appliances, there is a high demand to lower the cost of refrigerators, their replacement parts and maintenance fees. At the same time, durable refrigerators are desired because it is burdensome and costly to fix or replace the refrigerators. Nowadays, some consumers want a refrigerator that lasts for two decades.
One bottleneck to realize durable refrigerators is their defrost timer. A typical defrost timer is made of mechanical parts such as a motor, gears, cams and levers, which are designed to count time and turn off the evaporator at a preset time. Mechanical defrost timers are cost-effective due to the facts that they do not contain numerous components and each component is not an expensive part. However, their mechanical components make it difficult to produce a durable defrost timer that lasts over a decade. Because the mechanical components keep receiving forces and moving all the time, they are easily worn out. In addition, the mechanical defrost timer makes ticking or clicking noise, which can be heard in a quit room. Some people mind such noise in late night. Therefore, quiet defrost timers are required to make the refrigerator more quiet.